Hafler 200 bias

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#61
dh200 bias

Hi to all,

I'm new to the whole forum scene, so please bear that in mind. After about 25 years of happy dh200 listening, I am about to embark on some upgrades/ modifications as seen and discussed here and elsewhere. One thing I've seen mentioned is increasing the bias for better sound. While I don't wish to turn this amp into a space heater, I was wondering what the general feeling is of this tweak. If it is generally considered a good thing, could someone provide or point me to a step by step method to do this?

Most of the mods I'm about to make are beefing/cleaning up the PS and star grounding, new I/O connectors and silver wiring. I may tackle a few board level capacitor changes if I get brave and/or start saving the pennies for the Musical Concepts boards.

Any guidance for a relative novice would be greatly appreciated.
 
#62
appologies for earlier post

Just wanted to appologize for the earlier question on how to set the bias on the dh200. I had logged on for a quick peak at lunch and failed to notice that I was on the lst page of this thread. Most of my questions had already been addressed on an earlier page. Next time I'll only post when I have enough time.

I would be interested, however, in everyone's opinion of setting the bias higher. Would I just be asking for other problems or does this tweak make a hearable difference? I don't want to go crazy making tweaks that my 55 year old ears can barely hear.

Also, I was wondering when adding bypass caps to the electrolytics on the boards, would I use the same holes the present caps are in or do I need to drill new holes in the circuit trace on the board?
 
#63
Some say that upping the bias from 250 mA to around 350 mA improves the sound, especially in the mids and highs. However, a lot more heat is generated. The heat sinks are not sized large enough to tolerate a great increase in heat.

How good is the "improved" sound? It depends on your hearing and the resolution capability of your listening system. Also, some more technically trained would only increase bias while monitoring wave forms on an oscillosope to make sure that increasing bias does not increase distortion.

Personally I never go beyond 300 mA and then only when ambient room temperature is low -- like in the winter. I hear a slight improvement in the sound, but not so much that I can't live without it.

Also increased heat inside the amp can shorten the lifetime of other components so raising the bias is not without risk.

You can increase it slightly and listen and monitor the heat and if all goes well and you like the result, try raising it a little more. But, be careful.
 
#64
Re: appologies for earlier post

jmervin said:
Just wanted to appologize for the earlier question on how to set the bias on the dh200. I had logged on for a quick peak at lunch and failed to notice that I was on the lst page of this thread. Most of my questions had already been addressed on an earlier page. Next time I'll only post when I have enough time.

I would be interested, however, in everyone's opinion of setting the bias higher. Would I just be asking for other problems or does this tweak make a hearable difference? I don't want to go crazy making tweaks that my 55 year old ears can barely hear.

Also, I was wondering when adding bypass caps to the electrolytics on the boards, would I use the same holes the present caps are in or do I need to drill new holes in the circuit trace on the board?
Click to expand...

I have played a bit with bias on 2 DH-200 amps and this is what I discovered:
Increasing to about 300 ma or more has the consequence of inceasing the heatsink temperature and because of the temperature coefficient of these lateral mosfets, the bias will decrease after a while (a couple of hours of listening) to about 250 - 260 ma anyway. The fact is that 60Vdc is quite high for a 100 Wrms/8 ohma amp. This high voltage contributes to have more heat to dissipate. I have already asked Hafler tech department and they told me the optimum settings is about 260 ma. As Dick West said, if you use your amp in a warm environment the heatsink temperature will even get hotter. On my side I used about 275 ma with a reasonable heatsink temperature (just before being uncomfortable to the touch).
As for the sound, I have not really noticed the difference between 250 ma and 275 ma. Maybe with something like 350 or 400 ma but I would be worried about the temperature consequences.

For the bypass caps, you can use the solder side of the board since the bypass are low weight and only the solder is sufficient to secure them in position.

Good luck


Dick West said:
Some say that upping the bias from 250 mA to around 350 mA improves the sound, especially in the mids and highs. However, a lot more heat is generated. The heat sinks are not sized large enough to tolerate a great increase in heat.

How good is the "improved" sound? It depends on your hearing and the resolution capability of your listening system. Also, some more technically trained would only increase bias while monitoring wave forms on an oscillosope to make sure that increasing bias does not increase distortion.

Personally I never go beyond 300 mA and then only when ambient room temperature is low -- like in the winter. I hear a slight improvement in the sound, but not so much that I can't live without it.

Also increased heat inside the amp can shorten the lifetime of other components so raising the bias is not without risk.

You can increase it slightly and listen and monitor the heat and if all goes well and you like the result, try raising it a little more. But, be careful.
Click to expand...

Agree.
 
#65
Thanks

Thanks Dick and Fab for your input. I think for now I'll leave the bias alone and just upgrade the components. I like the sound already, so if the bias increase doesn't make an appreciable difference, it's not worth the extra heat generated to me. Thnaks again.

Jerry
 
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